1. Field
The present invention relates generally to radio frequency processing systems, methods, and articles of manufacture. More particularly, the invention relates to downconverting a received signal to an intermediate or baseband frequency. The systems, methods, and articles of manufacture may be employed in telecommunications, including uses in cellular access terminals.
2. Background
A modern communication system is expected to provide reliable data transmission for a variety of applications, such as voice and data applications. In a point-to-multipoint communications context, known communication systems are based on frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and perhaps other multiple access communication schemes.
A CDMA system may be designed to support one or more CDMA standards, such as (1) the “TIA/EIA-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (this standard with its enhanced revisions A and B may be referred to as the “IS-95 standard”), (2) the “TIA/EIA-98-C Recommended Minimum Standard for Dual-Mode Wideband Spread Spectrum Cellular Mobile Station” (the “IS-98 standard”), (3) the standard sponsored by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents known as the “W-CDMA standard,” (4) the standard sponsored by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents including “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,” and the “TIA/EIA/IS-856 cdma2000 High Rate Packet Data Air Interface Specification” (the “cdma2000 standard” collectively), (5) the 1xEV-DO standard, and (6) certain other standards. The standards expressly listed above are incorporated by reference as if fully set forth herein, including annexes, appendices, and other attachments.
Receivers of access terminals generally include mixers for converting the received radio frequency signals to a baseband or a lower intermediate frequency (IF), for subsequent processing at that frequency.
In full duplex systems such as CDMA and W-CDMA systems operating in accordance with the standards mentioned above, a receiver of an access terminal is subject to interference from the terminal's own transmitter, whose transmit signal leaks through a duplexor to an input of a low noise amplifier (LNA) at the receiver's front end. The transmitter's leakage may be modulated with an external jammer signal (any other interfering signal), causing cross-modulation products to degrade the receiver's sensitivity. In a zero-IF receiver, for example, the transmitter signal leakage may also produce second harmonic products, falling within the baseband at mixer output, thereby contaminating the desired signal in the receive path.
To deal with the transmitter signal leakage, the LNA may be designed to meet stringent cross modulation (linearity) requirements. This typically necessitates large current consumption in the LNA. Also, an inter-stage surface acoustic wave (SAW) filter may be interposed between the LNA and the mixer input to attenuate the transmitter leakage ahead of the mixer, thereby reducing the stringency of the requirements applicable to the local oscillator path and to components following the down-converting mixer. Inclusion of a SAW filter in an access terminal, however, may increase cost and size of the terminal.
A number of sometimes competing criteria affects the design of communication devices, such as cellular access terminal receivers and GPS receivers. The criteria includes size, weight, power consumption, and noise figure. Therefore, there is a need in the art for radio frequency receivers with improved noise figures. There is also a need in the art for radio frequency receivers with reduced size and reduced weight. There is a further need in the art for receivers with reduced power consumption.